Abstract Blastic plasmacytoid dendritic cell neoplasm (BPDCN) is a hematologic malignancy believed to originate from plasmacytoid dendritic cells (pDCs), the immune cells responsible for producing type 1 interferons during infection. Nearly all patients with BPDCN have prominent skin involvement, with cutaneous infiltration occupying the dermis and subcutis. One half of patients present with BPDCN cells only in the skin, with no evidence of disease elsewhere. Because normal pDCs are rare or absent in cutaneous sites, and they only traffic to the skin after activation by pathogen or inflammation, our aim was to determine if a microorganism is associated with BPDCN. We performed RNA sequencing in BPDCN skin and bone marrow, with cutaneous T-cell lymphoma (CTCL) and normal skin as controls. GATK-PathSeq was used to identify known microbial sequences. Bacterial reads in BPDCN skin were components of normal flora and did not distinguish BPDCN from controls. We then developed a new computational tool, virID (Viral Identification and Discovery; https://github.com/jnoms/virID), for identification of microbial-associated reads remaining unassigned after GATK-PathSeq. We found no evidence for a known or novel virus in BPDCN skin or bone marrow, despite confirming that virID could identify Merkel cell polyomavirus in Merkel cell carcinoma, human papillomavirus in head and neck squamous cell carcinoma, and Kaposi’s sarcoma herpesvirus in Kaposi’s sarcoma in a blinded fashion. Thus, at the level of sensitivity used here, we found no clear pathogen linked to BPDCN.